The present invention relates to an ink film thickness distribution correction method and apparatus for correcting an ink film thickness distribution formed in an ink roller group in an ink supply apparatus.
FIG. 12 shows the main part of an inker (ink supply apparatus) in a printing unit of each color in a web offset printing press. In FIG. 12, the inker includes an ink fountain 1, an ink 2 stored in the ink fountain 1, an ink fountain roller 3, a plurality of ink fountain keys 4 (4-1 to 4-n) juxtaposed in the axial direction of the ink fountain roller 3, an ink ductor roller 5, an ink roller group 6, a printing plate 7, and a plate cylinder 8 on which the printing plate 7 is mounted. An image is printed on the printing plate 7. The ink fountain 1, ink fountain roller 3, ink fountain keys 4, ink ductor roller 5, and ink roller group 6 form an ink supply path for supplying ink in the ink fountain 1 to the printing plate 7.
In the ink supply apparatus, the ink 2 in the ink fountain 1 is supplied to the ink fountain roller 3 by adjusting the opening degrees of the ink fountain keys 4-1 to 4-n. The ink supplied to the ink fountain roller 3 is supplied to the printing plate 7 via the ink roller group 6 by the ink feed operation of the ink ductor roller 5. The ink supplied to the printing plate 7 is printed on a printing sheet via a blanket cylinder (not shown). Note that ink form rollers 6-1 to 6-4 in contact with the printing plate 7 are arranged at the end of the ink flow path of the ink roller group 6.
FIG. 13 shows a printing product printed by the printing press. A band-shaped color bar 9-2 is printed in a margin except for an image region 9-1. In general four-color printing, the color bar 9-2 is formed from regions S1 to Sn each including black, cyan, magenta, and yellow density measurement patches (solid patches with 100% dot area). The regions S1 to Sn correspond to the key zones of the ink fountain keys 4-1 to 4-n in printing units of respective colors in the printing press.
[Color Matching]
Reference density values are set in advance for printing units of respective colors. More specifically, reference density values are set in advance for black, cyan, magenta, and yellow. When printing a printing product 9, color matching work is performed to make the density values of the respective colors match their reference density values. An ink supply amount control apparatus performs this color matching work during test printing or final printing based on the densities of density measurement patches 9a (9a1, 9a2, 9a3, and 9a4) of the respective colors on the color bar 9-2 printed on the printing product 9.
For example, the region S1 on the printing product 9 will be explained as a representative. The density value of the density measurement patch 9a of each color of the printing product 9 obtained by test printing or final printing is measured. The density difference between the measured density value of each color and a preset reference density value of this color is obtained. From the obtained density difference of each color, the correction amount (correction amount of the ink supply amount to the region S1) of the opening ratio of the ink fountain key 4-1 in the printing unit of this color is obtained. The opening ratio of the ink fountain key 4-1 in the printing unit of each color is adjusted using the obtained correction amount as a feedback amount.
As for the regions S2 to Sn, the correction amounts (correction amounts of the ink supply amounts to the regions S2 to Sn) of the opening ratios of the ink fountain keys 4-2 to 4-n in the printing units of the respective colors are obtained in the same way. The opening ratios of the ink fountain keys 4-2 to 4-n in the printing units of the respective colors are adjusted using the obtained correction amounts as feedback amounts. Immediately after the opening ratios of the ink fountain keys 4-1 to 4-n are adjusted, printing restarts. This operation is repeated until the density values of the respective colors reach their reference density values.
However, in this ink supply amount adjustment method, when the density of a printing product becomes excessively high during test printing or final printing, excessive ink in the ink supply apparatus hardly decreases by only decreasing the opening ratio of the ink fountain key. Many wasted sheets are generated, wasting printing materials. In addition, time is taken, decreasing the operation rate.
To efficiently correct an ink film thickness distribution in the ink supply apparatus during test printing or final printing, there have been proposed an ink film thickness correction method disclosed in Japanese Patent Laid-Open No. 10-16193 (literature 1), and an ink film thickness control method disclosed in Japanese Patent Laid-Open No. 11-188844 (literature 2).
[Literature 1 (Ink-Removing+Pre-Inking 2)]
In the ink film thickness correction method described in literature 1, when correcting an ink film thickness distribution in the ink supply apparatus during test printing or final printing, the ink feed operation of the ink ductor roller 5 is stopped. In this state, a predetermined number of sheets are printed (blank sheet printing), decreasing ink in the ink supply apparatus (ink-decrease). A minimum ink film thickness distribution Ma (see FIG. 14A) which thins from the upstream side to downstream side of the ink roller group 6 and is required during printing, that is, an ink film thickness distribution Ma corresponding to an image-free portion of the printing plate 7 remains. Then, a modified ink film thickness distribution Mb (see FIG. 14B) is superposed on the remaining ink film thickness distribution Ma (pre-inking 2).
[Literature 2 (Pre-inking (−)=“Ink-Returning to Fountain”+Pre-Inking 1)]
In the ink film thickness control method described in literature 2, when correcting an ink film thickness distribution in the ink supply apparatus during test printing or final printing, the opening ratios of all the ink fountain keys 4-1 to 4-n are set to 0. In this state, the ink feed operation of the ink ductor roller 5 is performed by a predetermined number of times, returning all ink remaining in the ink supply apparatus to the ink fountain 1 (“Ink-returning to fountain”). After that, a minimum ink film thickness distribution Ma (see FIG. 14A) required during printing is formed in the ink roller group 6 (first step of pre-inking 1). A modified ink film thickness distribution Mb (see FIG. 14B) is superposed on the formed ink film thickness distribution Ma (second step of pre-inking 1).
However, the ink film thickness control method described in literature 1 wastes sheets because blank sheet printing is executed when leaving the ink film thickness distribution Ma on the ink roller group 6.
The ink film thickness control method described in literature 2 takes time because all ink on the ink roller group 6 is returned to the ink fountain 1 and an ink film thickness distribution (Ma+Mb) modified from 0 is formed. In this method, emulsified ink (ink kneaded with damping water) is returned to the ink fountain 1. A printing trouble may occur, wasting printing materials.